Here is the full list of all 73 Left 4 Dead achievements. It takes around 60-80 hours to unlock all of the achievements on Windows.
Kill 53,595 Infected.
Kill 1000 Infected with a mounted machine gun.
Kill 5,359 Infected in the Crash Course campaign.
Dole out 5000 total Survivor damage as a Special Infected.
Earn a Gold medal on every official Survival level.
Earn at least a Silver medal on every official Survival level.
Earn at least a Bronze medal on every official Survival level.
Complete one round of Survival on The Last Stand.
Earn a Gold medal in Survival mode on any official level.
Earn a Silver medal in Survival mode on any official level.
Earn a Bronze medal in Survival mode on any official level.
Kill a Special Infected with an exploding barrel.
Sneak past all Witches in a campaign without disturbing one.
Play an entire campaign with no Survivors taking friendly fire damage.
Finish a campaign without ever using a first aid kit on yourself.
Survive a campaign without being incapacitated.
Survive a campaign with no Survivors taking damage from Special Infected.
All Survivors complete a campaign without being vomited on.
Survive an entire campaign using only pistols.
Save another Survivor from a Special Infected while on the ground.
Leave a safe room to save an incapped teammate and bring them back safely.
No Survivors take damage after contacting the rescue vehicle.
Win a Versus campaign of Crash Course.
As A Tank hit 20 Survivors with a car in the Crash Course campaign.
Complete "The Sacrifice".
Have Bill sacrifice himself for the team.
Your team wipes all Survivors after the escape vehicle has opened in Crash Course.
Kill all four Survivors on one life as a Tank.
Single-handedly kill a Tank.
Your team incapacitates three Survivors within five seconds.
Stun a Tank with an explosion in the Crash Course campaign.
As an Infected, incap a Survivor who has entered and left a safe room.
Cover four Survivors with Boomer bile at once.
Revive 50 incapacitated Survivors.
Catch a rare strain of infection, then pass it on to someone else.
Pounce a Survivor for 25 points of damage in the Crash Course campaign.
Pounce two different Survivors on one life as a Hunter.
Drag a Survivor 100 feet with your tongue as a Smoker.
Constrict two Survivors on one life as a Smoker.
Smoker pull a bile-covered Survivor until you hold him during Crash Course.
Kill a Tank without it dealing any damage to a Survivor.
Have all 3 generators running at once in "The Sacrifice" finale.
As a Special Infected, incap someone who is trying to sacrifice themselves.
Survive all campaigns on Expert.
Survive the Blood Harvest campaign.
Survive the Dead Air campaign.
Heal 25 Survivors with a first aid kit.
Shove a Boomer and then kill him without him splashing on anyone.
Survive the Death Toll campaign.
Set 101 Infected on fire.
Survive the Crash Course Campaign.
You or another Survivor take no damage after being vomited on by a Boomer.
Restart the generator within 30 seconds of it shutting off.
Protect any Survivor from an attacking Infected 50 times.
Light a Witch with a Molotov.
Give pain pills to 10 Survivors.
Kill 10 Smokers as they are pulling helpless Survivors.
Survive any campaign on Expert.
Survive the No Mercy campaign.
Beat a campaign with all 4 Survivors.
Make 100 headshot kills.
Kill a Witch with a single headshot.
Kill a Smoker who has grabbed you with his tongue.
Rescue a Survivor trapped in a closet.
Blow up 20 Infected in a single explosion.
Headshot a Hunter while he's leaping.
Kill a Witch without any Survivor taking damage from her.
Shove a Hunter off of a pinned and helpless Survivor.
Rescue a Survivor from a Smoker's tongue before he takes damage.
Light a Tank with a Molotov.
Heal a fellow Survivor when your own health is below 10.
Punch a Hunter as he is pouncing.
Kill an Infected with a single blow from behind.